Plastic fuel tanks are gaining popularity because of the reduction in the weight and the high production efficiency which they provide. As high density polyethylene (HDPE) which is preferred as the material for blow molding lacks the barrier property against the permeation of hydrocarbon (HC), a gasoline tank made solely of HDPE is not acceptable for use because of the concern for polluting the atmosphere. Therefore, it has been customary to incorporate a fuel permeation barrier layer made of ethylene vinyl alcohol (EVOH) having a favorable barrier property against fuel permeation into the plastic fuel tank.
In such a fuel tank, the filler neck, vent valve and other components are typically molded by separate molding processes, and have to be welded to the tank main body. In particular, such component parts are not only required to be welded to the tank main body with an adequate mechanical strength but also without compromising the fuel permeation barrier property of the fuel tank. As a technology for eliminating discontinuities in the barrier layer that could be the cause of fuel permeation, it was proposed in
JP2002-235624A to form an outwardly directed dome-shaped protuberance in a tank main body of a fuel tank made by laminating a weldable material layer made of HDPE and a HC barrier layer, form an opening in the tank main body by cutting off the dome-shaped protuberance along a plane cutting surface to expose the weldable material layer and the HC barrier layer in a concentric manner, and weld a component part to the exposed cutting surface such that the HC barrier of the tank main body abuts the HC barrier provided on the component part in a corresponding manner.
The HDPE used for manufacturing the tank main body often consists of recycled material containing a certain amount of impurities, instead of virgin material. In such a case, some effort is required to ensure the welding between the tank main body and the component part to be carried out in a reliable manner. Based on this consideration, it is conceivable to provide an additional weldable material layer made of pure HDPE around the existing weldable material layer made of recycled HDPE so that the component part may be welded to the additional weldable material layer
However, when the above mentioned welding method is applied to a tank main body of a fuel tank including a recycled material layer, the surface area of the weldable layer exposed on the cut surface produced by the cutting of the outwardly directed dome-shaped protuberance is so small that an adequate welding strength cannot be achieved.